Do Soybeans Respond to Nitrogen Fertilizer?

Soybeans require a tremendous amount of nitrogen to achieve high yields. Much of the nitrogen comes from atmospheric fixation but a considerable amount is taken from soil reserves. From 40 to 75% of the N in a mature soybean plant comes from the soil, not through fixation. If this is the case why not simply apply commercial nitrogen fertilizer to maximize yields. One of the problems with applying soil nitrogen is that plants stop atmospheric fixation and use the most available nitrogen first, namely that in the soil. So applying commercial fertilizer simply replaces "free" nitrogen the plant would take from the atmosphere with commercial fertilizer. Hundreds of experiments have been conducted applying various nitrogen fertilizer sources at different growth stages to avoid this problem and achieve more yield. One approach we tried was to deep band ESN (slow release) nitrogen 6 inches below the row at planting time. The concept was that soybeans would nodulate normally during the first part of the growing season before the root accessed the commercial fertilizer. When demand for nitrogen increased as the plants grew the root would reach the reserve of deep banded ESN and yields would increase. We applied 100 lbs/acre of actual N as ESN at five replicated sites in 2009. The yield results were so disappointing that we abandoned this approach. See table #1.

Table #1. Soybean Response to Deep Banded ESN at Seeding Time

	Untreated	100 lbs ESN (6" depth)
Average Yield Response	49.9 bu/ac	50.6 bu/ac
Soybean yield response of 5 replicated trials conducted in Wellington, Middlesex, Kent, Perth and Elgin counties in 2009.

Since demand for nitrogen spikes at podset and pod fill (R3) perhaps adding nitrogen at R3 would be the most beneficial, so recent work has focused on this timing. Fifty six trials conducted in Nebraska (cropwatch.unl.edu) showed a yield increases with 27 lb/ac N applied of 2.5 bu/ac in south central Nebraska, 1.6 bu/ac in the northeast, and 0.3 bu/ac (not significant) in southeast Nebraska, when yields were over 60 bu/ac. Applying 54 lb/ac N compared with applying 27 lb/ac N or when adding 4.5 lb/ac sulfur (S) did not add more yield. These results agree with other studies that have found inconsistent yield benefits but have generally showed that applying nitrogen at the R3-R4 growth stage is more likely when yields are over 60 bu/ac. If yields are below 60 bu/ac achieving an economic yield reponse has been very inconsistent.

In 2011 we have set up a number of experiments to see if applying slow release nitrogen at seeding time along with sulphur will provide economic yield benefits in no-till fields with heavy corn residue. Plants do look healthier and darker green but yield results will be what counts. See picture #1 We are also testing a slow release foliar product at the R3 growth stage. Stay tuned for those results.

Figure 1. Soybeans require tremendous amount of nitrogen